Various types of compact fluorescent lamps use a discharge vessel which is formed of one or more U-bent glass tubes which define a discharge path. If only one glass tube is used, electrodes can be melt-sealed into the two open ends of the U-bent tubes; a plurality of such glass tubes can be assembled, with tubular cross connections between the respective glass tubes to form a continuous discharge path. Electrodes are then melt-sealed in the open ends of the legs of the U-tubes which form the terminal ends of the continous discharge path. Such tubes, customarily, are formed of essentially parallel legs which are then connected at the U-bend by an essentially flat cross-connecting portion so that, in side view, the tubes have an essentially rectangular outer appearance. At the bend, that is, at the transition region of the cross-connecting portion of the U-tube, the spacing of the outer surface with respect to the inner surface is larger than the diameter of the longitudinal legs of the U-tube.
Compact fluorescent lamps, as described, are also patented and reference is made to the U.S. Pat. No. 4,481,442, Albrecht et al, as well as U.S. Pat. No. 4,853,591, Klein et al, assigned to the assignee of the present application, the disclosures of which are hereby incorporated by reference, for detailed descriptions. The cross-connecting portions of these prior art lamps are, usually, somewhat elliptical, and have a major diameter parallel to the legs of the lamp which is slightly larger than that of the diameter of the generally circular leg portions. The entire structure is made by blow-molding, after the respective regions of a continuous glass tube is heated and bent. At the apex of the bend, and particularly at the outer corners of the bend, the thickness of the glass tube has a tendency to vary substantially. Thus, the cross-connecting portion is more sensitive with respect to shocks and vibration than the longitudinal leg portions. It has been found that in mass production of the discharge tubes, in handling, and prior to impact resistant packaging, it is unavoidable that the individual, already bent discharge tubes hit against each other. This may lead to damage and destruction of the discharge tubes, particularly if one cross-connecting portion of a bent discharge tube impinges against the cross-connecting portion at the U-bend of an adjacent discharge tube.